1. Field of the Invention
This invention relates to an optical pickup for recording and/or reproducing the information for an optical disc, designed to optically record and/or reproduce the information, such as a magneto-optical disc or a phase-change disc.
2. Description of Related Art
As the format for the next-generation optical disc, such a format employing a light source with a wavelength on the order of 400 to 410 nm, by blue purple semiconductor laser, and an objective lens with a numerical aperture (NA) of 0.85, is currently used. As an optical disc, illuminated by laser light with the wavelength on the order of 405 nm, such an optical disc having a cover layer of a thin thickness of, for example, 0.1 mm, for protecting a signal recording layer, has been proposed.
For providing an optical pickup, used with the next-generation optical disc, such a one which is compatible with optical discs of different formats, such as state-of-the-art CD (Compact Disc) or DVD (Digital Versatile Disc), is desirable. That is, an optical pickup and a recording and/or reproducing apparatus, compatible with optical discs different in disc structures and, concomitantly, in laser specifications, are needed.
As apparatus compatible with optical discs of different formats, there are those having plural optical systems and in which the respective objective lenses are switched from one wavelength to another. However, a changeover mechanism for plural sorts of objective lenses, complex in structure, is required, thus raising costs. On the other hand, since the actuator becomes bulky in size, it is difficult to reduce the size of the apparatus.
There is also a two wavelength compatible optical pickup in which part of the optical system, such as an objective lens, is co-owned (for example, see Patent Publication 1, Japanese Laid-Open Patent Publication 2002-236253). In an optical pickup, adapted for coping with optical discs, different in formats, there is raised a problem that spherical aberration is produced due to the difference in the wavelengths of the laser light radiated for respective optical discs or in the thickness of the cover layers of the discs, thus affecting recording and/or reproducing characteristics.
As the optical pickup having means for correcting the spherical aberration, there is such an optical pickup in which a collimator lens is moved along the optical axis. In this optical pickup, there is raised such a problem that spherical aberration is produced due to the difference in wavelengths of the laser light beams radiated to the optical discs, and in the thickness of the cover layers of the discs, thus affecting recording and/or reproducing characteristics.
As the optical pickup, having means for correcting the spherical aberration, there is such an optical pickup in which a collimator lens is moved in the direction of the optical axis. In such optical pickup, the spherical aberration for the light beams radiated from different light sources is corrected by displacing the collimator lens. With this optical pickup, the optical discs of different formats are coped with by adjusting the locations of the light source units, provided at different positions, and by displacing the collimator lens. However, should this configuration be adapted to a three wavelength compatible optical pickup, it is necessary to provide a complicated mechanism of, for example, a three-wavelength optical axis combining device, to render it more difficult to reduce the size of the apparatus.
If, in an attempt to reduce the size of the three wavelength compatible optical pickup, light beams are radiated from a radiating unit, arranged at approximately the same light source unit, and which is adapted for radiating the light beams, the location of the light source unit cannot be adjusted, so that, in case the refractive index of the glass is varied with wavelengths and hence the focal length of the collimator lens is changed, an optimum position relationship between a light emitting point and the collimator lens differs from one wavelength to the next.
As may be seen from the foregoing, it is extremely difficult to correct the spherical aberration of the optical pickup, adapted for realization of the three wavelength compatibility of the next-generation optical disc, DVD and the CD, and to reduce the size thereof simultaneously.